Pulse to rectangular wave converter



March 30, 1948.

L. 1.. LAKATOS PULSE TO RECTANGULARv WAVE CONVERTER 2 Sheets-Sheet 1 Filed March 30, 1945 DEC/{191E625 Zhwentor louds- (Ittorneg 155967701701 /7 P015! P155100 IW/ March 30, 1948. LAKATOS 2,438,638

PULSE TO RECTANGULAR WAVE CONVERTER Fil ed March 5'0, 194; 2 Sheets-Sheet 2 lentil. Za/zafqg ?atented Mar. 30, 1948 PULSE TO RECTANGULAR WAVE CONVERTER Louis L. Lakatos, Bala Cynwyd, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application March 30, 1943, Serial No. 481,176

9 Claims. 1

My invention relates to apparatus for utilizing or for indicating the reception of electrical pulses of short duration which recur at different repetition rates and particularly to apparatus which is effective to increase the energy content of such pulses.

The invention is useful in pulse-echo recognition systems, for example, where the received pulses are of only a few micro-seconds duration and where their repetition rate may be any one of a large number of preassigned repetition rates. One of the problems in designing such equipment is to obtain an output ignal from the receiver which contains suflicicnt energy at the repetition rate of the received pulses to operate the associated equipment such as a decoder and/or a recorder or to obtain a strong audiblesignal.

An object of the invention is to provide an improved method of and means for converting periodic short electrical pulses into a-correspondin periodic signal of increased energy content.

A further object of the invention is to provide improved apparatus for converting short electrical pulses recurring at a. certain frequency into an approximately square wave signal having the same fundamental frequency as said repetition frequency and of increased energy content at this frequency.

In a preferred embodiment of the invention the short electrical pulses are supplied through an amplifier to a-relaxation circuitcomprising a pair of tubes which are direct-current connected plate-to-grid in multivibrator fashion and which blockand unblockalternately. One of the plateto-grid connections includes a coupling capacitor shunted by a resistor, the capacitor and resistor having a suitable time constant to permit partial charge of the capacitor between successive received pulses. In the absence of received pulses, the coupling capacitor .has a direct-current charge thereon, having been charged by a flow of grid current. The other plate-to-grid connection includes a resistor only. .Upon reception of the first short pulse, the first tube is unblocked and the second tube is blocked and held blocked by the coupling capacitor for a short time. Mean while, the coupling capacitor discharges until the second tube is unblocked at which time the first tube is again blocked. Meanwhile the coupling capacitor receives an additional charge. Upon reception of the next short pulse the cycle of operation is repeated.

The periods available for charge and discharge of the coupling capacitor are interdependent and, as a result, after a few Cycles of operation the two periods are approximately equal whereby a substantially square wave is produced within the operating range of the apparatus regardless of the repetition rate of the received pulses. The circuit constants are such that the relaxation circuit always goes through oneful] cycle of opera- Figure l is a circuit diagram of a preferred embodiment of the invention,

Figure 2 is a group of graphs illustrating the results obtained with the apparatus of Fig. 1,

Figure 3 is a group of graphs that are referred to in explaining the operation of the invention,

Figure 4 is a group of graphs illustrating the operation of the circuit of Fig. 1 for the first few cycles when it is functioning according to the invention, and I Figures 5 and 6 are graphs that are referred to in explaining theinvention.

Referring to Fig. 1, the short received pulses indicated at ID are supplied through a vacuum tube VI to a relaxation circuit comprising a pair of vacuum tub-es V2 and V3. In this particular embodiment of the invention the pulses II) are applied to the tube VI with positive polarity and each pulse is immediately followed by a short negative pulse due to the grid-leak biasing action of the grid capacitor CI and the grid-leak resistor RI in the input circuit of tube VI. This negative pulse, which appears reversed in polarity at the anode of the tube VI, is the pulse that triggers the relaxation circuit V2V3. This method of applying triggering pulses to the circuit V2V3 is not an essential feature of the invention but it has certain advantages and will be discussed in detail hereinafter.

Referring more particularly to the circuit comprising the tubes V2 and V3, the anode of tube V2 is connected to the grid of tube V3 through a capacitor C which is shunted by a resistor R. The anode of tube V3 is connected to the grid of .tube V2 through a resistor I I. Operating voltage is supplied to the anodes of tubes V2 and V3 through anode resistors I2 and I3, respectively. The cathodes of tubes V2 and V3 are connected to ground through a common cathode biasing resistor I4.

The short triggering pulses appearing on the anode of the tube VI are impressed upon the grid of the tube V2 through a direct-current connection I6. Operating voltageis, applied to the anode 3 V of the tube VI through the anode resistor 13, and the coupling resistor l l. r In the absence of incoming pulses III, the relative voltage drops in anode resistor 13 and coupling resistor II and in cathode resistor I 4 will be such that tlregridoi tube V2 will be sufiicient ly negative with respect to its associated cathode to block the tube v2. The anode 'of tube v2 is now at its maximum positive potential and the 1 tube V3 is conducting since the RC coupling holds its grid positive with respectt'o its cathode.

The capacitor C has a D.-C. charge because of the grid current flow in the tube V3.

Upon the reception of a positive pulse It, the j grid of tube VI draws current and the small grid capacitor Cl is charged. Atthe end of pulse ID, capacitor Cl discharges through the grid resistor Rl to put a short negative pulse on the a grid of tube VI, thus making the plateof tube vl go mprepositiveto apply a positive-pulse to the r makes tub eVi. conduct- :sitive wh reby the grid posi ve and the anode of snar eqs'i ve- T s change in e voltageqf flute v3 is fed back to the vi third h the coup ling resistor ll; on the tube V3 is almost V3, asaiaig.s i s eteg d t at which time the reverse action takes place whereby the if and is held at cut-off hit "v'gfiiowii thrbug thecathcae resistor: It. Meanwhilet-he depict 'g'ridcurre t in tube vs applie anadditional charge to capacitor C.

wheh thenxt'puise I'm-booms, "the. tube v2 is again unblocked and the cycle of operation 'repeats. i1

The discharge and charge periods for successive cycles ofi op'eration "are iinlik'e and changing. in

duration foit-fsevral iiulsetp'eriods or cycles-as illustratedfinfil ig. "During the. first cycle the chargingrperiodQTz, butduring the second :cycle the discharge' periodT'siis' "short compared with thegchargihg, period 154 :Duringfthe third cycle the discharge period. -T5:a'gain is longer than-the chargin'gj eriodk'le but. it willbe noted that by thistim'e the-two periods are-:more nearly equal. Aite bqut. fi e c es. :for e mple, t e. i cha e an c a ge p r odsar :Qi. b tant lly .Qons nt ra qn. and are so. nea ,e ualfiha 1 he wa e orms? he. ppt ut, w ve 5, is. f i ly close to the of symmetrical square wave.

ath 1pr ti smi i of h e p en thisaction"takesjplace regardlgs of the Irepeti tion. rate of the receivedpulses ll], Thisfact is m ssage i 371%..- was: Ma net 01 P1 1 Ii} are shown as occurring at, two difiere'nt rates. However, the'output'w'aye l5 is appreximetei syiml siiwa quaie wav in each instanceand each s'quarewaye has theisame.repetitionrate as '1 thato'f thpul'se's l'0 beingapplied to thecircuit. j

It may be noted that, withapparatus aving given, circuiti 'constants, if" the 'r'epetition rate of the receiyed'pulses l0 is t'ooIlow, the apparatus will not functiohas described abo'veehd aslillus trated inFig 1. Instead, itwill operate as shown iniFi g. 3bedaiuse theI'coupling' fEapacitor. C will e"'a"'full.'charge'before the centages" of a r1311 half cycle :1

sumed in Fig. 3, the discharge time for capacitor C is substantially constant whereas if the pulse period is shorter, as is assumed in Figs. 2 and 4,

the discharge time for capacitor C is a function of the charging time of capacitor C and the apparatus will function as desired. I

, The reason that my circuit operates as illust'r'ated in Figs. 2 and 4 may be seen more clearly by referring to the graphs of Figs. 5 and 6. The

graph l in Fig. 5 shows the discharge time of the coupling capacitor C plotted against its charging time. As here used, discharge time means the timefollowing an input pulse which is required-'to discharge capacitor C to a potential which wiiiperm t the two tubes v2 and v3 to pu ses it and they are 'pldttedfora'circhitliaving. the cons ants indicated in r g; In Fig. 1 the capacity and resistance ival-ues, given by way of example, ar'eindicat'ed in .microfara'cfis, m'icromioifofa rads, ohms and meg V. ,7 e-"T e i s'sa and ordihates are fractions 0 pulse period; i. e,

- ractions orth pert nent-guises 'lpi f Each'of the graphs sl'1ci 'v s ('by its ordinate values) the fraction of a 'icrtainfpiilse period required for the discharge time ef eapeeiter "C "y enin the preceding pulse period the fraction for the pe ridd-N reduir" w and assume t at the re et tion rate iS IOOQcycIes per second. The fractionfoftheneXtcycleor period-jlV ilreqiiire'd frpr discharge-1 of: capacitor {Cfis indicated'on the-graphfibythearroWat'the right,

this ordinate value being a2=0 .19. .Now taking d2 as the abscissa value, fit will {be -seen that Ior the mesa per'h, 'th'e' friti of-the period required for discharge arcane" 'tor 'cjjis a'a oa'. isi i a i e 'r hddt he. fi ihjm s er djand hat the nv r ent; d" il s. holds fIQr: ra hs 2. nd wer rq, trava t'm yhg 'ts thet'atit h rpw ej es;th sifi ea'va ei .s ar t g the ra i ji i i sms ss en ecorered., Forexample;overalfrquncy jn fof 5.00.c;jp.fs.

10. 090 cji gisasj ercent; 15,000

90 percent.

t med-sem For "As previous-1y "statedfit "is n61; heeessaryte isease utilize the grid-leak biasing action at the input circuit of the tube VI for triggering the relaxation circuit V2V3. For example, the tube VI may be operated as a class A amplifier drawing no grid current, in which case the received pulses ID are applied thereto with negative polarity. The arrangement illustrated and previously described is preferred, however, as it is a simple way of making certain that the triggering pulse which appears on the grid of the tube V2 is of sufficient duration to give the circuit V2V3 time to switch to the alternate condition of tube blocking. It will be understood that by selecting properly the values of Cl and RI, the negative pulse immediately following a positive pulse may be given a width or duration greater than that of the pulse It.

It has been found that for best operation of the specific circuit described the amplitude of the input pulses should be limited to values between 3 volts and volts positive. While specifi circuit values have been given by way of example, the values are not critical and may, for instance, be changed individually by plus or minus 50 percent with little change in operation. The +B voltage may be from 150 volts to 250 volts, for example.

Mention may be made of the fact that if the tube V1 is omitted from the circuit, the remaining circuit comprising tubes V2 and V3 will oscillate as a multivibrator with the frequency of oscillation determined largely by the time constant of the circuit R0.

.The complete circuit comprising tubes VI, V2, and V3 may be utilized as an electronic frequency meter by measuring the alternating current voltage appearing across the capacitor C. This A.-C. voltage is a function of the repetition of the applied pulses and is substantially independent of the amplitude and of the wave form of the input wave within the limits of circuit operation. The peak value of the said A.-C. voltage decreases as the frequency of the input signal decreases.

I claim as my invention:

1. The method of operating a pair of crossconnected tubes wherein one of the cross-connections consists of a capacitor shunted by a resistor and wherein the time constant for both charge and discharge of said capacitor is long compared with the period between successive applied pulses, said method comprising applying regularly recurring short electrical pulses to said pair of tubes and initiating the discharge of said capacitor in response to the occurrence of a short pulse, causing said capacitor to charge partially in response to said capacitor discharging to a predetermined voltage value, and again initiating the discharge of said capacitor by the next succeeding short pulse while said capacitor is only partially charged whereby said charge and discharge periods tend to become equal after a few cycles of operation. I

2. Apparatus for converting periodically recurring short electrical pulses into pulses of longer duration recurring at the same rate as the short pulses, said apparatus comprising a pair of amplifier tubes having cathodes and having anodes and grids which are direct-current cross-connected, one of said connections including a series capacitor shunted by a resistor, the other of said connections being substantially purely resistive, and a cathode resistor common to said tubes, the time constant of said series capacitor and shunting resistor being long ompared with the period between successive short pulses.

3. in combination a pair of vacuum tubes each having a cathode, a grid and an anode, a cathode resistor common to said tubes, a coupling ca-- pacitor shunted by a resistor connected between the anode of one tube and the grid of the other tube, a coupling resistor only connected between the anode of said other tube and the grid of said one tube, and means for applying short periodically'recurring pulses of positive polarity to the grid of said one tube, the time constant of the charging circuit for said coupling capacitor being substantially greater than the period of said recurring pulses.

- 4. In' combination a pair of vacuum tubes each having a cathode, a grid and an anode, an anode resistor for each tube through which a direct-current voltage is applied to its anode, a cathode resistor common to said tubes, a coupling capacitor shunted by a resistor connected between the anode of one tube and the grid of the other tube, and a coupling resistor only connected between the anode of said other tube and the grid of said one tube, and means for applying short periodically recurring pulses of positive polarity to the grid of said one tube, the time constants of the charging circuit and of the discharge circuit for said coupling capacitor being substantially greater than the period of said recurring pulses.

5. In apparatus responsive to periodically recurring electrical signals, a pair of amplifier tubes each having an anode and a grid, said tubes having direct current connections from the anode of one tube to the grid of the other tube, the said connections from the anodeof one tube including a series capacitor shunted by a resistor, the said connection from the anode of the other tube being substantially purely resistive and the timeconstant of said series capacitor and shunting resistor being long compared with the period be-- tween said successive electrical signals, means formaintaining said one tube biased substantially to cut-ofi in the absence of a signal voltage on its: grid, and means for applying said electrical sig-* nals to the grid of said one tube.

6. Apparatus for converting periodically recurring short electrical pulses into pulses of longer duration recurring at the same rate as the; short pulses, said apparatus comprising a pair of amplifier tubes each having an anode and a. grid, said tubes having direct-current connections from the anode of one tube to the grid of the other tube, the said connection from the anode of one tube including a series capacitor shunted by a resistor, the said connection from,

the anode of the other tube being substantially purely resistive and the time constant of said series capacitor and shunting resistor being long compared with the period between said successive short pulses, means for maintaining said one tube biased substantially to cut-off in the absence of a signal voltage on its grid, and means for applying said short electrical pulses with positive polarity to the grid of said one tube.

7. In apparatus responsive to periodically recurring electrical signals, a pair of amplifier tubes each having an anode and a grid, said tubes having direct current connections from the anode of one tube to the grid of the other tube, the said connection from the anode of one tube including a series capacitor shunted by a resistor, the said connection from the anode of the other tube being substantially purely resistive and the time constant of said series capacitor and shunting resistor being long compared with the period between said successive electrical signals, means QASMQQ 1y to cutr-offimthe rabsgn e f a 81m volta e on t rid, and'means ox' ap l in said smart electrical pulses to the id of saislon? H139 w ereby the e app a s ac ss said' er fi ea aei tor an a t ating current mana e having a peak alue that decreas s a ism: mpet fion ate oi the electrical; signals increases. I

8. The invention according tq claim 3 whergin said means for applying short pulses inclugles an pl fi r tube ha ing, noutp el ct de that is direct-current connected to thegljid 9i said one tub.

9. The invention according to claim 5 wherein said means for maintaining saidgne tgbe b i as ed substantially to cut-off in the gbsencgoi a, signal voltage on its grid includes an ampyfigr tube havin an anode that is 119 m Hid.- of saidnpe uh direct-current connected U S L. LAKATOS. REFERENCES CITED The follgwin references are. of recQrd in th file 91", this patent:

Number Num 15 356,111 456,840

UNITED STATES PATENTS Great Britain Nov. 12, 1936 

